The present invention generally relates to transmission systems, and more particularly to a transmission system which transmits from a transmitter to a receiver a send data signal and a request to send signal by use of only a pair of transmission lines and carries out a carrier detection without being affected by a data pattern of the send data signal. In this regard, in the present disclosure all references to "carrier detection" refer to the detection of signal presence on the pair of transmission lines.
In a conventional transmission system employed in a local communication network, pairs of transmission lines are used exclusively for transmitting a request to send signal RTS and a send data signal SD from a transmitter, and for transmitting a received data signal RD and a carrier detect signal CD from a receiver. In other words, a large number of pairs of transmission line are required to transmit the signals RTS, SD, RD and CD.
In the conventional transmission system, because of the need to provide independent pairs of transmission lines for the signals RTS, SD, RD and CD, the provision of the lengthy and bulky transmission-line cables becomes a problem especially as the distance between the transmitter and the receiver becomes long. Accordingly, various transmission systems have been proposed to eliminate this problem.
For example, there is a previously proposed transmission system which uses only a pair of transmission lines to transmit the send data signal SD and the request to send signal RTS. According to this previously proposed transmission system, a carrier OFF state is detected when a logic value of the send data signal SD continues to be "0" for over a predetermined time. However, there is a problem in that the transmitting pattern of the send data signal SD becomes restricted. In addition, there is always a possibility that the data pattern of the send data signal SD becomes identical to the pattern (in this case, continuous "0" for over the predetermined time) which would be detected as the carrier OFF state.
Therefore, there is a demand for a transmission system which can transmit the signals RTS, SD, RD and CD by use of only a pair of transmission lines and will not generate the problems described above.